The skilled man knows of casting machines with a moving mold with rolls which are used for manufacturing, directly from a mass of molten metal, a continuous strip having a width which can be as large as several meters and a thickness of approximately one centimeter.
These machines are constituted principally:
(a) on the one hand by a liquid metal supply device comprising, in succession in the direction of flow of the metal:
(i) a furnace for keeping the metal in the liquid state, PA1 (ii) a runner for circulation equipped with a system adjusting the level and the rate of flow of the metal, PA1 (iii) a nozzle for the distribution of the metal having at its outlet end an opening of rectangular cross-section; PA1 it permits maximum productivity; PA1 it prevents all visible or invisible defects in solidification as it can operate continuously, and those resulting from stoppages of the machine; PA1 it reduces the labor needed for supervising the machine; PA1 it causes the machine to operate under better conditions as it no longer stops; PA1 it provides, via the calculator, a recording of the casting parameters which is a type of record card for the cast strip.
(b) on the other hand, a cooling and rolling device comprising two rolls of which the axles are parallel and spaced more or less from each other depending on the desired strip thickness. These rolls are provided at each of their ends with axial or journalled cylindrical extensions which engage via bearings in openings made in supporting or chock cross beams equipped with a gripping system and integral with two vertical columns forming the frame of the machine. These rolls are provided internally with a network of channels along which a coolant circulates and communicate with a motor which sets them into rotation in opposite directions.
These two devices are placed in relation to each other in such a way that the outlet cross-section of the nozzle is parallel to the axles of the rolls and is situated at a certain distance from the plane passing through these axles which is called the outlet plane.
During operation of the machine, the metal distributed by the nozzle fills the free space between the rolls along an arc of a circle contained between the plane of the outlet cross-section of the nozzle and the outlet plane of the cylinders.
Due to the action of the rolls, the metal cools, begins to solidify at a point known as the marsh or partly solidified metal, owing to the presence of a more or less viscous mixture of crystals and liquid situated at a distance from the plane of the outlet cross-section of the nozzle, generally known as the depth of partly solidified metal. The metal then solidifies completely and is driven towards the outlet plane of the rolls in a progressively restricted space where it is subjected to a rolling stress which gradually brings it to the desired thickness at the moment when it exits through the space between the rolls in the form of a strip which is then taken up by a roller.
This strip is then subjected to various mechanical and/or thermal treatments which lead to products such as thin sheet, for example, of which the mechanical properties: strength, yield stress, elongation, hardness, etc., are, in part, a function of the quality of the strip issuing from the casting machine.
It is, therefore, important to attempt to maintain a high quality from the beginning to the end of the casting of the strip. For this purpose, the machine must be made to operate under the conditions most favorable for obtaining such a result, even if it is used at its maximum speed.
A high quality necessitates the absence of defects such as cracks, fissures or break-out of metal as it leaves the rolls. Now several causes for the appearance of defects in strips manufactured from the casting machines under consideration are known. These are generally variations in certain operating factors such as as the temperature of the metal supplying the machine, the rate of flow of the coolant from the rolls, the state of the surface of the rolls connected with the conditions of lubrication, the composition of the cast metal, the level of the metal in the runner, etc.
Any variation of one of these factors can, depending on its magnitude, disturb the operation of the machine, that is to say produce an instability in the partly solidified metal at certain points of the strip which can reach the outlet plane of the rolls. This local instability of the partly solidified metal causes the appearance of greater or smaller defects on the strip, sometimes necessitating the rejection thereof.
The causes of defects are also connected to the speed of casting. In fact, it has been observed that beyond certain speeds, the stability of operation of the machine becomes more critical and more sensitive to certain hazards and, in particular, to variations in the operating factors listed above, which are translated by the increased frequency in the appearance of defects.
With regard to the causes of defects, a distinction should be made between those due to factors of which a variation can easily be detected such as the temperature, the level of metal, the flow rate of water. In these cases, it is easy to carry out automatic monitoring with an alarm permitting the operator of the machine to ready this variation rapidly. However, when dealing with factors such as the composition of the metal case or the surface state of the rolls, it seems to be fairly difficult, if not impossible, to be able to detect continuously the variation thereof. The operator can therefore react only on appearance of the defect, which causes a part of the strip to be rejected.
Whether the variation is detectable or not, it demands the permanent availability of the operator. However, owing to external constraints, the manufacturing staff cannot always react instantaneously or have their eye fixed on the strip, and a large defect such as a break-out sometimes appears and then necessitates a stoppage of the machine. This is why casting machines are frequently used at speeds far below their capacity so as to avoid the appearance of these defects due to possible variations in the operating factors.